Guilds are (so far) the only collaborative and social dimension in the game. Players apply for membership in a selected guild and, if accepted, contribute Stars to the guild. Access the guild part of the game through the web-globe icon that becomes available when you reach a specific level.
Guilds are made up of a maximum of 20 players. A player can apply for membership or leave a guild at any time. Membership acceptance is, per guild, either manual or automatic. Being possible for players to change guilds during a weekly challenge, it is possible to let more than 20 players contribute stars in a guild. Such deliberate player cycling is in general considered cheating.
Each guild is searchable, with a non-unique name and a guild description. Anyone can create a new guild, at the expense of XP. If you are looking for a guild or are a guild looking for members, you can visit the official guild forum available.
Advanced guilds may make alliances outside the game, in order to channel the best players into the guild that has the most chance at the top list, and to share strategy secrets exclusively.
The guilds participate in the Weekly challenge with mainly three purposes. As of update 1.4, players no longer need to be in a guild in order to participate in the weekly challenges, but not being a guild member excludes you from some activities and rewards.
Earn more rewards
The promise of more rewards may be the initial purpose to join a guild. There is a "Join or create a guild" Achievement as a first hint into exploring the guild feature.
Through acceptance and promotion mechanism, and through communication in the embedded chat, the guilds adds a social dimension to the game. Within the chat players can challenge each other and make friends. That human interaction enforce player loyalty to the game.
Advanced guilds usually also form a separate chat or forum for easier communication.
Boast your rank
There are leaderboards in the game, to show global and local top guilds. For the top 50 guilds this is a boasting opportunity and for some a very important goal. Within the guild the players also rank according to the stars collected in the active challenge.
There is a global and local top 50 leaderboard for personal star count too, but you likely do not need to be currently in a guild to show there.
There are three member types available as titles in the guild. Different types does not have any in-game advantages, such as better rewards, but control guild administration rights.
The creator of the guild (generally) becomes the Leader. The leader has the ability to accept new members, promote/demote members and to kick members from the guild. The leader can change the guild description.
It is currently not possible to change the leader of a guild. To work around that, the new leader needs to create a separate guild and have all members join that instead. When the leader is the only member (e.g. from kicking all others) the guild will be deleted if he leaves the guild. The leader can not leave the group unless empty.
If the leader just stops playing the game the guild will not be deleted. If it has an elder the guild can essentially function just as well as before, with the exception of the leader not collecting stars and occupying one of the member slots.
A promoted member becomes Elder. In addition to everything a Member can do, Elders have the ability to accept new members, promote members to Elder, demote Elders to Member, and to banish Elders or Members from the guild. An elder has the right to do this on both other elders and normal members, but it is not possible to demote or kick a leader from the guild.
The promotion can either be used as a means to assign a number of assistant leaders for administration purposes, or mainly seen as a way of assigning titles to esteemed members. In any case, the Elder has the given admin rights.
The base rank of a guild is Member. This member has no admin rights, they may:
Participate in group chat
Contribute stars to guild total
Receive reward from guild